This invention relates in general to electrical current measuring techniques and apparatuses and in particular, to a high voltage, direct current measuring technique and apparatus.
In certain applications, it is desirable to measure a direct current flowing through a conductor at a high voltage. For example, in a multi-capillary electrophoresis system it is desired to know the current in each of a multitude of capillaries electrically connected together in parallel. During the chemical electrophoresis operation, one end of each capillary is immersed in a common pool of conductive liquid or gel buffer along with an electrode connected to electrical ground, while the other end is immersed in a vial of buffer along with an electrode maintained at a high dc voltage, either positive or negative. Typically, such voltages may be in a range of 5,000 to 50,000 volts, while the current flowing through each of the capillaries may range between 1 to 100 microamps.
One technique for measuring current flowing through a conductor is to measure a voltage across a known resistance connected in series with the conductor, and calculate the current from the measured voltage and known resistance using Ohm's law. For digital processing and/or display purposes, it is desirable to digitize the measured analog voltage using an analog-to-digital converter. When doing so, the measured analog voltage is generally first scaled with respect to the full-scale range of the analog-to-digital converter to improve the accuracy of the conversion. Examples of such full-scale ranges are 0 to 5 volts, and 0 to 10 volts.
In a multi-capillary electrophoresis system, however, it is often impractical to measure a voltage across a known resistance at the electrical ground end of the capillaries since that end is immersed in a common pool of conductive liquid or gel buffer. Also, it is often impractical to measure a voltage across a known resistance at the high voltage end, because of the large voltage conversion or level shifting (e.g., up to 50,000 volts) required to scale the measured analog voltage down to the full-scale voltage range of a typical analog-to-digital converter (e.g., 0 to 5, or 0 to 10 volts).